1. Field of the Invention
The present invention relates to a gas concentration sensor for detecting the concentration of a specific gas component of a gas under measurement on the basis of a propagation time between transmission of an acoustic wave and reception of the acoustic wave, and more particularly to a gas concentration sensor that minimizes gas concentration measurement errors and measurement inability states caused by the adhesion of standing liquid within the sensor.
2. Description of the Related Art
Conventionally, a fuel supply system for supply of fuel from a fuel tank to an engine includes a first supply system which functions in the following manner. Fuel is pumped from the fuel tank by means of a pump and then sent to an injector through a fuel pipe. The fuel supply system further includes a second supply system which functions in the following manner. Fuel vapor generated within the fuel tank is temporarily adsorbed by a canister. Accumulated fuel vapor is purged from the canister and is sent as purge gas to an intake pipe.
In an engine equipped with the first and second supply systems, in addition to fuel injected from the injector, fuel vapor, such as purge gas, (hereinafter called “fuel vapor”) is supplied to a cylinder for combustion. In this combustion, control of air-fuel ratio is very important, in order to minimize the content of harmful gas, such as CO, HC (hydrocarbon), and NOx, in exhaust gas, which increases with deviation of an air-fuel ratio from a theoretical ideal value. To achieve control of the air-fuel ratio, the concentration of fuel vapor is measured with high accuracy, and the amount of fuel vapor and the amount of fuel injected from the injector are controlled on the basis of the measured values. Gas concentration sensors have hitherto been used as means for detecting concentration of fuel vapor; and an example of such a conventional gas concentration sensor is an ultrasonic wave gas concentration sensor, which is currently under development. The ultrasonic wave gas concentration sensor can determine the concentration of fuel vapor on the basis of a propagation time between transmission of an acoustic wave and reception of the reflected acoustic wave.
Such an ultrasonic wave gas concentration sensor is shown in Japanese Patent Application Laid-Open (kokai) No. H7-209259. This publication proposes a structure for mounting a gas concentration sensor for a vehicle, which structure enables accurate and efficient detection of gas concentration even when liquid is produced within the sensor as a result of condensation of fuel vapor or water vapor. As shown in FIG. 10, in the proposed structure, gas holes (inflow and outflow holes 15 and 16), which allow inflow and outflow of gas in the state in which the gas concentration sensor is mounted on a vehicle or an engine, are disposed in a lowermost portion in a measurement chamber 13 provided between an ultrasonic wave transmitting-receiving element 11 for transmitting and receiving an ultrasonic wave and a reflection wall 12. Use of this structure reduces adverse effects of liquid that is generated as a result of condensation of fuel vapor and water vapor within the sensor and liquid that is generated as a result of condensation of fuel vapor and water vapor outside the sensor and enters the sensor.
However, in the case where the above-described structure is employed, there would arise a problem that liquid generated as a result of condensation of fuel vapor, water vapor, etc. is apt to adhere, for a long period of time by surface tension, to the corner between a periphery of the ultrasonic wave transmitting-receiving element 11 and a wall surface of a container 18; the corner between a peripheral portion of a reflection wall 12 and the wall surface of the container 18; and a bottom portion of the wall surface of the container 18, which surrounds the measurement chamber 13.
For example, if standing liquid 14 adheres to the ultrasonic wave transmitting-receiving element 11, the standing liquid 14 hinders transmission and reception of an ultrasonic wave, thereby lowering, for example, the output, receiving sensitivity, and transmitting-receiving efficiency of the ultrasonic wave transmitting-receiving element 11. Here, the term standing liquid refers to liquid generated as a result of condensation of fuel vapor, water vapor, etc. within or outside the sensor and standing within the sensor (within the measurement chamber) without being drained out of the sensor.
Further, when standing liquid 14 adheres to the peripheral portion of the reflection wall 12, the ultrasonic wave received by the ultrasonic wave transmitting-receiving element 11 includes not only components reflected by the reflection wall 12, but also those reflected by the surface of the standing liquid 14. As a result, the ultrasonic wave transmission distance would be a distance L2, which is shorter than a true distance L1. In this state, because the output would be calculated as if the acoustic wave velocity were increased, the gas concentration would be calculated as being lower than a true value, with the result that an accurate value of the gas concentration cannot be obtained.
In practice, in a gas concentration sensor using an ultrasonic wave, a reflection wave to be received by an ultrasonic wave element (ultrasonic wave transmitting-receiving element) is attenuated by influences attributed to the material of a sensor housing, the shape of a wall surface of a measurement chamber in which the ultrasonic wave propagates, the surface shape of a reflection wall, the propagation distance of the ultrasonic wave, the frequency of the ultrasonic wave, the gas pressure, the gas temperature, and other factors. In view of the foregoing, a technique is employed in which a threshold level is set by use of a portion of a received wave in order to change the threshold level in accordance with the amplitude of the received wave, and propagation time is measured accurately by use of the changed threshold level. However, if a portion of a received wave to be used for setting the threshold level contains an improper-path wave propagated along a path other than a proper path, the resulting threshold level would deviate from the proper value, thereby causing errors in measuring the concentration of a gas.